Is my texture gone if I accidently clicked on "blank" in UV mapping?
Yes, unfortunately it's gone forever. Hopefully there will be a day where you can undo that, but in the meantime you should save your images often or paint your textures in another app. There are lots of decent texture painting softwares out there: Mudbox, Mari Indie, 3D Coat, Mari, Substance Painter, Quixel Suite, Bodypaint, and so on, and most of them have better layer management than Blender and care more about keeping your data intact.

If you pick Mudbox make sure to turn up Brush Sample Area under Painting in the prefs. The other apps have sane defaults. If you stick with Blender (which is actually not a bad choice) just make sure to constantly ask yourself, If Blender crashed right now or my image disappeared, would I cry?